The present invention relates to a system and method for controlling a turbogenerator/motor to provide automated or semi-automated transitions between grid following and stand-alone operating modes.
A permanent magnet generator/motor generally includes a rotor assembly having a plurality of equally spaced magnet poles of alternating polarity around the outer periphery of the rotor or, in more recent times, a solid structure of samarium cobalt or neodymium-iron-boron. The rotor is rotatable within a stator which generally includes a plurality of windings and magnetic poles of alternating polarity. In a generator mode, rotation of the rotor causes the permanent magnets to pass by the stator poles and coils and thereby induces an electric current to flow in each of the coils. Alternatively, if an electric current is passed through the stator coils, the energized coils will cause the rotor to rotate and thus the generator will perform as a motor.
One of the applications of a permanent magnet generator/motor is referred to as a turbogenerator which includes a power head mounted on the same shaft as the permanent magnet generator/motor, and also includes a combustor and recuperator. The turbogenerator power head would normally include a compressor, a gas turbine and a bearing rotor through which the permanent magnet generator/motor tie rod passes. The compressor is driven by the gas turbine which receives heated exhaust gases from the combustor supplied with preheated air from the recuperator.
A permanent magnet turbogenerator/motor can be utilized to provide electrical power for a wide range of utility, commercial, and industrial applications. While an individual permanent magnet turbogenerator may only generate 24 to 75 kilowatts, powerplants of up to 1000 kilowatts or greater are possible by linking numerous permanent magnet turbogenerator/motors together. Potential applications for these lightweight, low noise, low cost, environmentally friendly, and thermally efficient units include standby power, peak load shaving power, and remote location power, among others.
When operating in a grid connected mode, the turbogenerator system is generating power in parallel with a utility grid. In this mode, the system may act as a current source, or alternatively as a voltage source with series impedance. When generating power in parallel with the utility, various protective functions or features should be provided to assure the safety of utility workers and others who interface with the utility grid. For example, features should be provided to disconnect the turbogenerator system from the utility grid when abnormal conditions occur on the grid system, particularly when the section of the grid to which the turbogenerator system is connected becomes isolated from the remainder of the utility system. This condition is sometimes referred to as xe2x80x9cislanded operationxe2x80x9d and should be avoided because utility workers normally expect the voltage to collapse when they disconnect a grid section from the remainder of the utility. If this does not occur as expected, and the utility workers do not follow proper safety procedures such as measuring the voltage and applying grounds before touching conductors, islanded operation may result in a safety risk. Another safety risk is that a generator will start up and energize a de-energized section of the grid with which utility workers may be in contact.
When operating in stand alone mode, the turbogenerator system is generating power to supply loads that are isolated from the utility grid. The system will generally operate as a voltage source in this mode although other configurations are possible. In stand alone mode, the protective features used in grid connect mode should be disabled because the system is essentially operating in an islanded condition and energizes a de-energized line. However, various other features may be desirable, such as avoiding energizing a line that is already energized so that stand alone controls can not be activated when connected to a utility grid.
A gas turbine is an inherently limited thermal machine from the standpoint of its ability to change rapidly from one load state to a different load state. In terms of accepting increased loading, the gas turbine has a limited capability or slew rate which may depend upon the particular application. As one example, a turbogenerator may be able to support an increasing load at a rate of about two (2) kilowatts per second. This creates problems in many stand-alone applications where the load may be essentially instantaneously applied in as little as about one millisecond. Similar limitations apply to decreasing loads as well. When operating in a self-sustained manner, the gas turbine has a very large amount of stored energy, primarily in the form of heat stored in the associated recuperator. If the load is removed from the gas turbine too quickly, this stored energy could result in overspeeding the turbine.
As such, various energy storage and discharge systems have been developed. The energy storage and discharge system includes an ancillary electric storage device, such as a battery, connected to the generator controller through control electronics. Electrical energy can flow from the ancillary electric storage device to the turbogenerator controller during start up and increasing load and vice versa during self-sustained operation of the turbogenerator. To start the turbine, a microprocessor-based inverter connects to and supplies fixed current, variable voltage, variable frequency, AC power to the permanent magnet turbogenerator/motor, driving the permanent magnet turbogenerator/motor as a motor to accelerate the gas turbine. When utility power is unavailable, the ancillary electric storage device can provide the power required to start the turbogenerator. As the turbine accelerates to an appropriate light-off speed, spark and fuel are introduced and self-sustaining gas turbine operating conditions are reached. When transitioning between stand alone and grid connect operating modes, proper phase synchronization may be necessary to avoid damaging equipment associated with the protected load.
When a load transient occurs in stand alone mode, the gas turbine engine and the ancillary electric storage device provide the power required to successfully meet the transient. The output power control regulates a constant AC voltage and any load placed on the output will immediately require more power to maintain the same level of AC voltage output. As this occurs, the internal DC bus may start to droop. In response, the ancillary electric storage device control draws current out of the storage device to regulate the DC bus voltage. As turbogenerator system power output increases, the gas turbine engine controls respond by commanding the gas turbine engine to a higher speed. When the power generated by the gas turbine engine meets or exceeds the current load, the ancillary storage device starts to draw power from the DC bus for future storage. Proper management of the ancillary storage device is necessary to assure stored energy availability for use during stand alone starts where utility power is not available, during transitions between grid connect and stand alone operating modes, and to accommodate transient load increases while maintaining acceptable energy storage component life.
The present invention provides systems and methods for controlling a turbogenerator to facilitate switching between a grid following operating mode and a stand-alone operating mode. A dual mode controller may by an external controller which communicates with the primary turbogenerator/motor controller via an appropriate communications interface to selectively and automatically reconfigure a PWM inverter from a grid-connect mode to a stand-alone mode based on availability or condition of utility grid power. Control commands and data/status information may be communicated between the turbogenerator/motor controller and the dual mode controller via serial communication interfaces (RS-485, Ethernet), logic signals, relays, contacts, and the like. In one embodiment, the dual mode controller is integrated into the turbogenerator/controller and may include the same or similar contacts for backward compatibility.
The present invention provides a dual mode controller for a turbogenerator which includes a rotation sequence selector which can automatically monitor the rotation sequence of three phase power from a utility grid and use the same rotation sequence when transitioning between grid-connect and stand-alone mode modes. Alternatively, or in combination, the present invention allows a user/operator to select a rotation sequence while operating in a manual stand-alone mode which may include a positive or negative sequence, or may use the last measured sequence for the utility grid. A default rotation sequence, either positive, negative, or grid, is also provided for automatic transitions between the grid-connect and stand-alone mode operation. In one embodiment, the present invention also provides for phase reference angle synchronization to synchronize the reference angle in stand-alone mode with the utility grid prior to completing the transition to grid-connect mode.
The present invention preferably includes an auxiliary electrical power storage and discharge system, such as one or more batteries, for providing a stand-alone start of the turbogenerator/motor without connection to a utility grid, for providing power to service transient loads, and/or for providing power to a protected load during transitions between grid-connect and stand-alone operating modes. Appropriate management of the auxiliary electrical power and discharge system may include periodic top-up charges, equalization charges, and recharges to assure power availability.
The present invention includes various safety features while transitioning between grid connect and stand-alone operating modes. For example, if an attempt is made to operating in grid connect mode when the connected load is actually a stand-alone, then the system inhibits starting to avoid safety hazards. If an attempt is made to operate in stand-alone mode when the system is connected so the utility grid and the grid is energized, the system will not start. The invention prevents the system from being configured in the stand-alone mode when it is connected to a utility grid by providing an appropriate lock-out system. An isolation device contains an auxiliary low voltage contact that always has the same position as the main power contacts. The system interfaces with this contact via stand-alone and grid-connect enable lines. When the switch is closed, the stand-alone enable is clear and the grid-connect enable is set. The invention opens the isolation device if the turbogenerator system enters the stand-alone state where the output is energized by using state feedback signals provided by the turbogenerator.
The present invention provides a number of advantages relative to prior art strategies. For example, the present invention can provide automatic or semiautomatic transitions between grid-connect and stand-alone operating modes with only momentary breaks or interruptions in supplying power to a protected load. The ability for rotation selection control of three-phase power generation in addition to phase synchronization provides for automatic transitions from stand-alone modes to grid-connect operating modes. In addition, rotation sequence control may by used to prevent equipment damage otherwise caused by a rotation sequence reversal when switching to stand-alone mode.
The above advantages and other advantages, objects, and features of the present invention, will be readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.